Add SplitKit::isOriginalEndpoint and use it to force live range splitting to terminate.

An original endpoint is an instruction that killed or defined the original live
range before any live ranges were split.

When splitting global live ranges, avoid creating local live ranges without any
original endpoints. We may still create global live ranges without original
endpoints, but such a range won't be split again, and live range splitting still
terminates.

llvm-svn: 126151